Semiconductor device manufacturing processes include an etching process for etching a substrate by using a plasma. In this etching process, etching gas reacts with a layer to be etched or a base layer to produce a composite product on the substrate, and the composite product can remain thereon.
For example, a contact hole for connecting an electrode to a source or drain region of a MOS transistor is formed in a silicon oxide layer on a silicon substrate by etching in which a CF-based etching gas and oxygen gas is used.
FIG. 16 schematically shows a composite product 208a produced on the bottom of a contact hole 208. In FIG. 16, the components of the composite product 208a are also shown in a combined form. When a silicon oxide film 202 formed on a silicon layer 201 of a substrate 200 is etched by using a CF-based etching gas and an oxygen gas, an amorphous silicon layer 203 and the composite product 208a formed of a CF polymer 205 (containing carbon and fluorine) and a silicon oxide layer 204 are formed sequentially on the bottom of the contact hole 208. This is considered to be formed by the following process.
When the silicon oxide film 202 is etched to expose the surface of the silicon layer 201, as shown in FIG. 17B, the surface layer of the silicon layer 201 is changed by a plasma energy into an amorphous silicon layer 203. Further, the top portion of the amorphous silicon layer 203 is oxidized by a plasma of an oxygen gas to form the silicon oxide layer 204 (FIG. 17C). Thereafter, the CF polymer 205 containing carbon and fluorine is accumulated on the silicon oxide layer 204 to form the composite product (208a) (FIG. 17D). The composite product 208a increases contact resistance, thus reducing the yield of semiconductor devices. For this reason, the composite product 208a needs to be removed.
Conventionally, there is known a technique in which ashing is carried out by using an oxygen plasma to remove CF residues after completing etching process. However, the oxygen plasma cannot be used, because the use of the oxygen plasma increases an oxidation of the silicon layer 201 on the substrate 200.
Further, from results of various experiments, it has been found that the composite product 208a is not a simple laminate schematically shown in FIG. 17D. That is, the silicon oxide layer 204 may be a mixture of a silicon oxide 207 and a CF compound 206 as shown in FIG. 18A or a compound in which silicon, oxygen, carbon and fluorine are chemically bonded to each other as shown in FIG. 18B, which is in a very stable state. For this reason, the composite product 208a has not been able to be sufficiently removed by a process of cleaning the substrate with an organic solvent or an acidic solution, which is generally carried out after etching. Particularly, as the design rule of semiconductor devices continues to be further miniatured or the aspect ratio increases, the cleaning solution cannot be (sufficiently diffused into holes or trenches, and thus sufficient cleaning cannot be achieved.
The composite product 208a is a composite of organic and inorganic materials. Thus, if a process for removing the inorganic material is carried out, the organic material cannot be removed, or if a process for removing the organic material is carried out, the inorganic material cannot be removed. That is, the composite product 208a is considered a very troublesome residue. For this reason, developing a method of removing such residue from the semiconductor device has become a very pressing task.
Further, it has been suggested that, in a process for manufacturing a specific kind of device, as shown in FIG. 19A, etching is performed on a substrate in which a silicon oxide film 210 and a polysilicon film 211 are alternately laminated (for example, in two layers), through a resist film 215, thereby forming a recess 220.
In this case, a plasma of a halogen gas and a plasma of a gas containing carbon and fluorine are used to etch the polysilicon film 211 and the silicon oxide film 210, respectively. In the etching of the polysilicon film 211, a halogenated silicon oxide 213 containing halogen, silicon and oxygen is deposited on the sidewall of the recess 220. Meanwhile, in the etching of the silicon oxide film 210, a polymer 212 containing carbon and fluorine is deposited on the sidewall of the recess 220. As a result, as shown in FIG. 19B, a laminated product 214 laminated with the halogenated silicon oxide 213 and the polymer 212 is deposited on the sidewall of the recess 220.
The laminated product 214 also reduces the yield of semiconductor devices, and thus needs to be removed. However, the laminated product is difficult to remove reliably, because it is a stable material.
Japanese Patent Laid-open Application No. H4-83340 (particularly, page 2, light column, lines 23-44, and page 7, left column, pages 10-15) discloses a method of removing particles produced during etching of a substrate by using HF steam after cleaning the substrate with alcohol steam. However, the above-described inorganic/organic composite product is not mentioned therein.